The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some of such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
In most communication systems, several users share a common medium, such as an optical fiber or a radio path. Different multiple access methods have been developed to allow several users to simultaneously use a communication system efficiently. Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA) and Code Division Multiple Access (CDMA) are four multiple access methods that are widely used in wireless systems. In FDMA, users are separated in time domain. Transmissions of the users are separated by assigning the users different frequency bands. In OFDMA, different symbols of users are transmitted in parallel using many sub frequencies, thus increasing the spectral efficiency as compared with FDMA. In TDMA, users are separated in time domain. Each user is given a time slot, during which it can transmit using the entire channel bandwidth.
In CDMA, all users simultaneously share the entire available frequency band. Each user is assigned a unique spreading code. The codes allow a receiver to separate one user from the others although their channel symbols are transmitted simultaneously in the same frequency band. The codes used are selected in such a way that the simultaneously transmitted signals are orthogonal with each other. Thus, ideally, they do not interfere with each other.
In CDMA based communication systems, it is possible to use a so-called pilot channel in the transmission direction of base to subscriber equipment, i.e. in the downlink direction. A pilot channel is a signal which is transmitted with a specific PN (pseudo noise) spreading code and utilizing the same frequency band on which the actual traffic channels are situated, the pilot signal being distinguishable from them only on the basis of the spreading code. The pilot signal is a channel known and listened to by all subscriber equipment within the cell area, and it is used for example in power measurements and in the generation of a coherent phase reference. Each base station or sector of the system transmits its own pilot signal on the basis of which the subscriber equipment can distinguish the transmissions of different base stations from each other. In CDMA all base stations may transmit using the same frequency band.
Different variants of CDMA have been proposed. For example, in systems based on Interim Standard 95 (IS-95), a pilot signal is transmitted continuously in addition to forward traffic channels. In Evolution-Data Optimized or Evolution-Data only (EVDO) based systems, a pilot signal is not transmitted continuously. In EVDO, the transmission consists of slots comprising half slots having 1024 chips. The pilot signal is transmitted in the middle of each half slot in 96 chips.
For user equipment or a device monitoring the system the fast and reliable acquisition of the pilot signal is of utmost importance as it has an effect on the operation efficiency.